1. Field of the Invention
This invention relates to a light transparent, moisture barrier film composite. More particularly, this invention relates to a light transparent, thin film composite comprising layers of silicon nitride and fluorocarbon polymer which is useful as a moisture barrier for a moisture sensitive on-cell tester, a process for making said barrier and to an electrochemical cell having a moisture sensitive on-cell tester and said barrier.
2. Background of the Disclosure
The use of cell condition testers, such as thermochromic voltage testers, to visually indicate the condition of an electrochemical cell, commonly referred to as a battery, has become very popular and provides a value added advantage to the battery manufacturer and to the consumer. These testers are used with primary electrochemical cells, although they can also be used by consumers to test the condition of a secondary or rechargeable electrochemical cell if desired. The most popular tester presently in use is a thermochromic material in contact with an electrical resistance element which forms an integral part of a battery package in which the batteries are alkaline primary cells. The user places the terminals of the cell between the contacts of the tester and squeezes the contact ends of the tester to make electrical contact with the cell terminals which are the ends of the cell. The resistance element of the tester is heated in proportion to the cell voltage and the thermochromic material provides a qualitative indication of the condition of the cell over a range indicating "good" or "replace". This kind of tester is disclosed, for example, in U.S. Pat. No. 4,723,656. An integral, thermochromic package tester which can also be removed from the package is disclosed in U.S. Pat. No. 5,188,231. More recently, on-cell testers have been developed in which the cell condition indicator is an integral part of the cell label. These on-cell testers include both the thermochromic type and a new, electrochemical type of tester. An example of a thermochromic type of on-cell tester is disclosed in European Patent Publication No. 0 523 901 A1, the disclosure of which is incorporated herein by reference. Unlike the thermochromic type which employs a resistance element to produce heat and which can therefore not be permanently attached to the terminals of the cell without continuously discharging it, the new electrochemical type does not draw current from the cell and can therefore be permanently attached to the terminals of the cell without discharging the cell. This new type of tester is disclosed in U.S. Pat. Nos. 5,250,905 and 5,339,024 the disclosures of which are incorporated herein by reference. As is disclosed in U.S. Pat. No. 5,355,089 some electrochemical types of on-cell condition testers employ hygroscopic or otherwise moisture sensitive electrolyte compositions and means are necessary to prevent moisture from reaching the electrolyte which will impair the effectiveness of the tester. This patent discloses a number of solutions to this problem, the best of which is mica. However, although relatively inexpensive, mica is not available in long ribbons or other forms which permit it to be rolled into a roll of mica which is needed for economically viable commercial production methods.